Remove crypto::ECPrivateKey::IsSupported.

We no longer need to worry about the system NSS on decrepit Linux
distributions. The only platform where ECPrivateKey is backed by NSS is iOS and
we control that one.

BUG=519504

Review URL: https://codereview.chromium.org/1408813002

Cr-Commit-Position: refs/heads/master@{#354379}
7 files changed